1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly to transflective liquid crystal display devices.
2. Description of the Prior Art
Liquid crystal display (LCD) devices are in wide use as display devices capable of reducing the overall size, weight and thickness of electronic apparatuses in which they are employed. In general, LCD devices are divided into two categories—transmissive LCD devices and reflective LCD devices—according to whether the display uses an included or an external light source.
A transmissive LCD device displays images using light from a back light device, and is usable under any ambient light conditions. Because the transmissive LCD device requires a back light having high brightness, it has high power consumption. Further, the back light device cannot be used for a long time.
Unlike the transmissive LCD device, a reflective LCD device utilizes ambient light beams from a natural light source or from an external artificial light source. The reflective LCD device can be used for a long time. However, the reflective LCD device is useless when the weather is unfavorable or when the external light source is not available.
To overcome the problems described above, a transflective LCD device has been developed. The transflective LCD device can compensate for the respective shortcomings of the reflective LCD device and the transmissive LCD device. That is, the transflective LCD device can selectively provide a reflective or transmissive mode, depending on the prevailing needs of users.
FIG. 8 is a schematic cross-sectional view of part of a conventional transflective LCD device 1. For the sake of convenience, just one sub-pixel portion of the transflective LCD device 1 is shown. The transflective LCD device 1 includes an upper plate 10 having a color filter 101, a lower plate 12 spaced apart from the upper plate 10, a liquid crystal layer 11 between the upper plate 10 and the lower plate 12, and a back light 13 disposed below the lower plate 12.
Referring also to FIG. 9, in the transflective LCD device 1, a conventional color filter 101 is employed. The color filter 101 includes an upper transparent substrate 1011, a color filter layer 1012, and a transparent electrode 1013. The color filter layer 1012 includes a plurality of black matrix units 1015 regularly disposed on the upper transparent substrate 1011, and color units 1014 covering the black matrix units 1015. The color units 1014 are divided into red “R,” green “G” and blue “B” color units 1014. In the upper plate 10, the color filter layer 1012 is formed on a bottom surface of the transparent substrate 1011, and the upper transparent electrode 1013 is formed on a bottom of the color filter layer 1012. The upper transparent electrode 1013 serves as a common electrode. In addition, a half wave plate 102 is formed as a retardation film on a top surface of the transparent substrate 1011, and an upper polarizer 103 is formed on the half wave plate 102.
In the lower plate 12, an insulating layer 122 is formed on a top surface of a lower transparent substrate 121, and a lower transparent electrode 123 is formed on the insulating layer 122. A passivation layer 124 is formed on the lower transparent electrode 123, and a reflective electrode 125 is formed on the passivation layer 124. A transmitting hole 126 is defined through the passivation layer 124 and the reflective electrode 125. A lower polarizer 120 is formed on a bottom surface of the lower transparent substrate 121.
The transflective LCD device 1 has a transmissive portion “T” that corresponds to a portion of the lower transparent electrode 123 exposed via the transmitting hole 126, and a pair of reflective portions “R” that correspond to the reflective electrode 125. The transmissive portion “T” has a first cell gap “a” between the upper transparent electrode 1013 and the lower transparent electrode 123. The reflective portions “R” have a second cell gap “b” between the upper transparent electrode 1013 and the reflective electrode 125. The first cell gap “a” is configured to be larger than the second cell gap “b,” such that incident light rays have the same efficiency for the transmissive and reflective modes. Specifically, the first cell gap “a” is preferably about twice as large as the second cell gap “b.”
In the reflective mode, an ambient light ray “d” from an external light source such as natural sunlight passes through the upper polarizer 103, the half wave plate 102, the color filter 101 and the liquid crystal layer 11 in that order, and is then reflected by the reflective electrode 125 to pass back through the liquid crystal layer 11, the color filter 101, the half wave plate 102 and the upper polarizer 103 in that order. That is, the ambient light ray “d” passes through the color filter 101 twice.
In the transmissive mode, an incident light ray “c” from the back light 13 passes through the lower polarizer 120, the transparent substrate 121, the insulating layer 122, the lower transparent electrode 123, the liquid crystal layer 11, the color filter 101, the half wave plate 102 and the upper polarizer 103 in that order. That is, the incident light ray “c” passes through the color filter 101 only once.
The light ray “c” is only colored once by the color filter 101 in the transmissive mode, but the light ray “d” is colored twice by the color filter 101 in the reflective mode. Thus, in the transflective LCD device 1, the reflective mode has a better color purity than the transmissive mode. That is, there is a difference in color purity as between the reflective mode and the transmissive mode.
For the above reasons, an improved transflective LCD having high color purity is desired.